The use of "metallocene" catalysts for the polymerization of ethylene--i.e. catalysts which contain two cyclopentadienyl ligands--has been investigated for some 40 years, as evidenced by a series of patents which include U.S. Pat. No. 2,827,466 (Breslow, from a filing date of 27 Sep. 1955); U.S. Pat. No. 3,231,550 (Manyik, from a filing date of 4 Jun. 1964); and U.S. Pat. No. 4,542,199 (Kaminsky et al). Kaminsky et al discovered that certain aluminoxanes may activate zirconium-based metallocenes in a manner that produces a "single site" catalyst with spectacular productivity--a significant discovery which led to a large increase in research in this area of catalysis. Of particular relevance to the present invention, much investigation has been done on monocyclopentadienyl transition metal catalysts which are sometimes also referred to as "pseudo-metallocenes".
Although zirconium-based bis-metallocenes generally provide superior catalyst productivity in comparison to titanium-based metallocenes, the reverse is generally true with monocyclopentadienyl pseudo-metallocenes. This preference for titanium-based pseudo-metallocenes for ethylene polymerization is strongly reflected in the patent literature:
A. U.S. Pat. Nos. 5,055,438; 5,096,867; 5,227,440 These patents (which are assigned to Exxon) teach that so-called "constrained geometry" catalysts having one Cp ligand, one amido ligand and a bridging ligand between the Cp and amido, exhibit good productivity. The experimental data illustrate this claim for titanium-based complexes. There are no experimental data demonstrating the equivalence of the analogous zirconium complexes. PA1 B. U.S. Pat. Nos. 5,064,802; 5,132,380 These patents, which are assigned to the Dow Chemical Company, provide experimental data which are similar to that provided in the above Exxon patents (i.e. the Dow patents provide experimental data which exhibit the desirable properties of "constrained geometry", monocyclopentadienyl titanium complexes but there are no experimental data which demonstrates the equivalence of the analogous zirconium complexes). PA1 C. U.S. Pat. Nos. 5, 194,532 and 5,312,938 teach the preparation of cyclopentadienyl Ti (amido).sub.3 and indenyl Ti (amido).sub.3 complexes and ethylene polymerizations conducted in the presence and absence of methylaluminoxane in some of the complexes. There are no experimental data relating to the monocyclopentadienyl zirconium analogue. PA1 D. U.S. Pat. No. 5,324,698 (assigned to Neste Oy) claims the use of a monocyclopentadienyl titanium trichloride catalyst deposited on a magnesium chloride/electron donor support. This catalyst is active in the presence of ordinary aluminum alkyls (i.e. aluminoxanes are not required to activate this catalyst, which is highly desirable in view of the high cost of methylaluminoxane). There is no suggestion that the zirconium analogue has any utility. PA1 An olefin polymerization catalyst comprising essential elements (i), (ii) and (iii): PA1 C1 the catalyst support must be particulate; PA1 C2 a comparatively large amount of aluminoxane must be deposited on the support so as to prepare a catalyst precursor having from 200 to 500 milligrams of aluminoxane per gram of the particulate catalyst support; PA1 C3 the aluminum to zirconium molar ratio of the final supported catalyst must be at least 150/1, which means that a very small amount of zirconium is used to prepare the catalyst; and PA1 C4 the zirconium must be provided in the form of monocyclopentadienyl trihalide (preferably monocyclopentadienyl trichloride). PA1 (a) the simple filling of the pore volume of the support (i.e. some pores may be completely filled with aluminoxane); and PA1 (b) blocking the top opening of a pore (i.e. if a pore is envisioned as a cylindrical or conical container--for example, a drinking glass--then the anchoring of aluminoxane at only the top of the pore may effectively occupy the total pore volume by making the bottom of the pore non-available). PA1 2.1 The aluminoxane is thought to form an association complex with the monocyclopentadienyl zirconium catalyst species, thereby keeping the zirconium in the presence of the support (i.e. the high level of aluminoxane relative to the level of zirconium is believed to keep the zirconium associated with the support). PA1 2.2 The large amount of aluminoxane which is deposited on the support reduces the effective porosity of the support. In turn, this is believed to produce a smoother morphology with less open pores. In turn, this is believed to reduce the probability that the active zirconium catalyst will penetrate into the pores of the catalyst. This lower amount of internally deposited zirconium is thought to reduce the tendency of the supported catalyst to "fracture" during polymerization, which is thought to mitigate reactor fouling problems stemming from "fractured" catalyst particles. PA1 2.3 The high Al/Zr ratio on the supported catalyst eliminates the need to add extra uncomplexed or "free" aluminoxane to the reactor. (Note: the addition of extra, uncomplexed aluminoxane is highly undesirable and should not be done in the polymerization method of this invention.)
When a metallocene or a pseudometallocene catalyst is employed in a gas phase polymerization, it is highly preferred to use the catalyst in a supported form. It is also highly preferred that the catalyst does not cause reactor fouling. The art of preparing catalysts which do not lead to gas phase reactor fouling is not adequately understood, though it is generally accepted that the catalytic material should be very well anchored to the support so as to reduce the incidence of fouling resulting from the deposition of catalyst or cocatalyst which has dissociated from the support.
The preparation of supported metallocene and pseudo metallocene catalysts is reported in many U.S. patents including: U.S. Pat. No. 4,935,397 ("Chang", to Exxon); U.S. Pat. No. 4,808,561 ("Welborn", to Exxon); U.S. Pat. No. 5,240,894 ("Burkhardt" to Exxon); and the aforementioned Neste Oy patent, U.S. Pat. No. 5,324,698.
It is an object of this invention to provide a non-reactor fouling, supported pseudometallocene catalyst having good activity for ethylene co-(polymerization).